Scott R. Kennedy

Porous broadband antireflection coating by glancing angle deposition

We deposited graded-index SiO2 films using glancing angle deposition to produce high-transmission antireflection coatings on glass. Because of the accurate control over the thin-film microstructure provided by this technique, we were able to create graded densities with a Gaussian profile resulting in transmission values greater than 99.9% for a single-layer interface with bandwidths up to 460 nm. The graded-index layer also provides low reflectance at nonnormal angles of incidence with transmission values degrading little for incidence angles up to 30 degrees.

Three-dimensional square spiral photonic crystal nanostructures by glancing angle deposition

We demonstrate the fabrication of a three-dimensional photonic band gap nanostructure using the unique Glancing Angle Deposition (GLAD) thin film technique. This innovative photonic structure is based on a tetragonal, square spiral symmetry found in FCC and diamond lattices and can be implemented in a virtual single-step fabrication process using GLAD. A highly porous film with engineered nanostructure results from the implementation of advanced substrate motion during the deposition, and three-dimensional periodicity required for a photonic crystal can be obtained when a square spiral staircase is fabricated on a tetragonally seeded substrate. We present successful fabrication of three-dimensional square spiral structures along with some examples of the unique capabilities of these nanostructured thin films.

Fabrication of Periodic Arrays of Nanoscale Square Helices

We demonstrate fabrication of periodic arrays of nanometre scale square helices, with potential applications in three-dimensional photonic bandgap (PBG) materials. Processing is performed using a thin film deposition method known as Glancing Angle Deposition (GLAD). Through advanced substrate motion, this technique allows for controlled growth of square helices in a variety of inorganic materials. Organization of the helices into periodic twodimensional geometries is achieved by prepatterning the substrate surface using electron beam lithography. The regular turns of the helices yield periodicity in the third dimension, perpendicular to the substrate. We present studies of tetragonal and trigonal arrays of silicon helices, with lattice constants as low as 300 nm. By deliberately adding or leaving out seeds in the substrate pattern, we have succeeded in engineering line defects. Our periodic nanoscale structure closely matches an ideal photonic band gap architecture, as recently proposed by Toader and John. While our fabrication technique is simpler than most suggested PBG schemes, it is highly versatile. A wide range of materials can be used for GLAD, manipulation of lattice constant and helix pitch ensures optical tunability, and the GLAD films are robust to micromachining.

Fabrication of square spiral photonic crystals by glancing angle deposition

Using Glancing Angle Deposition, a novel thin film deposition technique, it is possible to fabricate complex, periodic structures suited for applications in photonic band gap crystals. In comparison to complex lithography processes used to produce conventional structures on the scale of several nanometers, GLAD is ideally suited to a virtual single step deposition, producing a novel tetragonal square spiral crystal structure and having a large predicted band gap of up to 15%.

Microfabrication of chiral optic materials and devices

Chiral thin films have been demonstrated to have significant optical activity and device applications for gratings, filters, retarders and optical switches. These helically nanostructured films may be microfabricated onto silicon or other substrates utilizing the Glancing Angle Deposition (GLAD) technique with various nanostructures such as helices, chevrons, or polygonal spirals. GLAD is a simple one-step process that enables ready integration of these structures onto optical chips. As proposed by Toader and John, the GLAD technique can be used to fabricate large bandwidth photonic crystals based on the diamond lattice. This structure yields a predicted photonic bandgap as much as 15% of the gap center frequency. Moreover, the corresponding inverse square spiral structure is predicted to have a photonic bandgap as much as 24% of the gap center frequency. We report the details of basic chiral thin film fabrication and calibration. We will also discuss optical characteristics of the chiral films such as the optical rotatory power. Finally, we present the results of our efforts to fabricate square spiral and inverse square spiral structures.

Optical performance of porous TiO2 chiral thin films

Porous thin film structures have been fabricated by physical vapor deposition at an incident flux angle that was typically greater than 80°. This deposition technique, often called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. Modification of the deposition parameters was used to control the porosity, the handedness, and the pitch of the helical structure. The high porosity of the GLAD film (>50%) permits fluids, and in particular liquid crystals (LC), to be incorporated into the voids of the nanostructure. We present the results of a study assessing the effect of film material, chiral morphology, and liquid crystalline material on the optical performance of helical GLAD films. Films fabricated from TiO2, a high refractive index material, exhibited strong optical rotation of linearly polarized light and selective reflection of circularly polarized light. By increasing the number of turns of the helix the chiral optical response was enhanced, and by tailoring the pitch of the helical columns, the wavelength-dependence of the reflection band was tuned to preferentially reflect red, green, or blue light.

Optical properties of chiral thin films fabricated by glancing angle deposition

Glancing angle deposition (GLAD), developed recently by Robbie and Brett, is an advanced technique of thin film deposition that can produce porous thin films with columnar microstructural features controllable on a ten nanometer scale. GLAD combines highly oblique angle deposition with computer controlled substrate motion to allow engineering of thin film microstructure for a diverse range of applications. Of particular promise for optical applications are chiral thin film morphologies, including films fabricated by GLAD possessing helical microstructure. Previous optical characterization has demonstrated rotation of the plane of polarization in these films. In this work, circularly polarized spectroscopic transmission measurements on helical GLAD films have shown selective reflection/scattering of the circular polarization which matches the handedness of the film, with the helical pitch controlling the peak wavelength. The geometric properties of films fabricated with GLAD, such as film density, helical rise angle, and helical radius can be controlled independently and easily allowing optical properties to be tailored as desired.